EAS and RFID systems are typically utilized to protect and track assets. In an EAS system, an interrogation zone may be established at the perimeter, e.g. at an exit area, of a protected area such as a retail store. The interrogation zone is established by an antenna or antennas positioned adjacent to the interrogation zone. The antenna(s) establish an electromagnetic field of sufficient strength and uniformity within the interrogation zone. EAS markers are attached to each asset to be protected. When an article is properly purchased or otherwise authorized for removal from the protected area, the EAS marker is either removed or deactivated.
If the EAS marker is not removed or deactivated, the electromagnetic field causes a response from the EAS marker in the interrogation zone. An antenna acting as a receiver detects the EAS marker's response indicating an active marker is in the interrogation zone. An associated controller provides an indication of this condition, e.g., an audio alarm, such that appropriate action can be taken to prevent unauthorized removal of the item.
An RFID system utilizes an RFID marker to track articles for various purposes such as inventory. The RFID marker stores data associated with the article. An RFID reader may scan for RFID markers by transmitting an interrogation signal at a known frequency. RFID markers may respond to the interrogation signal with a response signal containing, for example, data associated with the article or an RFID marker ID. The RFID reader detects the response signal and decodes the data or the RFID marker ID. The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID marker pass. A fixed reader may be configured as an antenna located in a pedestal similar to an EAS system.
It is advantageous in both EAS and RFID systems to establish a sufficiently strong and uniform magnetic field within the interrogation zone in order to provide for reliable marker detection. To provide such a magnetic field, magnetic core antennas have been utilized. A magnetic core antenna typically includes a long core of magnetic material over which a winding is disposed. The winding includes a conductor such as a wire conductor or copper ribbon that is uniformly disposed about the length of the core to form a coil. The coil, which is an inductive element, may be connected to a discrete capacitor to form a resonant circuit. When a transmitter is connected to this resonant circuit, current flows through the winding generating a magnetic field in the core and in the region around the core antenna.
The magnetic field induced in the core material by the current flowing through the winding increases proportionately with the current level through the winding and the number of turns of the winding (ampere-turns). The magnetic field intensity that projects outside the core, e.g., into the interrogation zone of an EAS system, is a function of the intensity of the magnetic field in the magnetic core and the distribution of the magnetic field along the length of the core. However, the intensity of the magnetic field in the magnetic core tends to decrease at the end portions of the core due to self-demagnetization of the core. This results in a decrease in the utilization of the core, and, consequently, a lower magnetic field about the core of the antenna.
Also, maximum field generation from an antenna occurs when the ampere-turns delivered to the antenna core is maximized. The ampere-turns associated with a particular antenna may be adjusted by adding or subtracting turns from the antenna winding. High power antennas typically require a low number of turns. In many situations, however, it becomes impractical to reduce the number of turns due to physical limitations in achieving good coupling to large core structures with a low number of turns. Therefore, an impedance transforming device, e.g., a transformer, is often utilized between the transmitter and the antenna. The impedance transforming device is, however, an additional and expensive component. When the impedance transforming device is a transformer, additional problems may occur such as the introduction of additional resonant tank circuits with magnetizing inductance of the transformer in the equivalent circuit and the generation of high voltage spikes in the transformer secondary.
In addition, magnetic core antenna assemblies have been constructed with magnetic materials such as ferrite or powdered iron. For shorter core antenna lengths, the cores may be molded or pressed as a single piece. However for longer core antenna lengths, it is difficult to manufacture cores in a single piece. Hence, such longer core antennas are typically constructed by stacking smaller core components in an end-to-end fashion to achieve a desired length. A longitudinal clamping force is then applied to the two ends of the core assembly. As the length of the core assembly increases, the longitudinal clamping force necessarily increases creating greater stress on the core components.
In such longer core assemblies, it is desirable to minimize air gaps between the contacting surfaces of the individual core components so that the magnetic flux can pass from one high permeability core component to another without crossing a low permeability air gap. Minimizing such air gaps between the contacting surfaces of the core components helps to maintain minimum reluctance of the core assembly. When utilized in an EAS system, this helps the core antenna assembly to achieve a high magnetic field in the interrogation zone.
Such air gaps between the contacting surfaces of the individual core components can be caused by mechanical stresses that cause the core antenna assembly to bend from its original straight position. Since the core components are typically brittle materials, e.g., ceramic magnetic materials, such stress forces can result in damage, e.g., chipping, to the core material at the comers of the end to end joints causing air gaps. This can occur during shipping and installation of such core assemblies.
Accordingly, there is a need for a high efficiency magnetic core antenna. There is also a need for an apparatus and method of controlling the impedance of a core antenna for maximizing power transfer to the antenna without a separate impedance transforming device. There is a further need for a core assembly and construction method to provide improved core component coupling to overcome the above deficiencies in the prior art.